In large liquid tanks, in particular fuel and oil tanks at refineries and the like, which are capable of holding large amounts of oil products, use is frequently made of a floating roof which floats on the liquid in the tank and is therefore displaceable in a vertical direction. Thus, the floating roof is capable of following the level of a liquid (oil product) when the liquid is discharged from or filled into the tank. Floating roofs of this type are used for preventing leakage of vapors and gases from the tank into the atmosphere and ingress of e.g. rainwater from the surroundings into the liquid. Typically, the prevention of leakage and ingress is enhanced by a sealing arrangement fitted along a perimeter of the floating roof for providing sealing and sliding contact with an inner wall of the tank. Further, using a roof that floats on the liquid enables minimizing a space between the liquid and the roof and thereby minimizing the amount of gases and liquid in vapor form in this space. In case of fuel and oil tanks, the environment on top of the floating roof is a hazardous or potentially hazardous environment.
Floating roofs for these purposes are usually manufactured as large steel structures with float means (pontoons) and have a weight in the order of a hundred tons and a diameter of tens of meters. With regard to size and environmental aspects, it is important to monitor the normal operation and undisturbed floating of the floating roof, such that disturbance thereof is identified at an early stage. Consequently, it is also important that any interruption of the monitoring of the floating roof is limited.
Different situations of disturbance to normal operation and floating have been observed in the past.
In filling of the tank, part of the floating roof could get stuck to the inner wall of the tank. As the filling proceeds the floating roof will be partially flooded by liquid, with a potentially hazardous situation as a result, should gas or vapor so produced be explosive or otherwise harmful. The roof structure braking could lead to an even more severe result.
In discharging of the tank, part of the floating roof could get stuck to the inner wall of the tank. As the discharging proceeds considerable amounts of air could enter the space between the liquid and the floating roof. In the subsequent event of a roof collapse, an explosive atmosphere could form above the oil and the collapsed roof.
Regulations tend to become stricter to minimize evaporation of liquid from floating roof tanks. This has resulted in floating roof designs which give a higher friction in the sealing arrangement between the perimeter of the floating roof and the inner wall of the tank. This increase in friction could increase the risk of a roof getting stuck.
Abnormal function and floating may similarly occur if a large amount of rain or snow is present or unevenly distributed on the roof. This could cause the roof to sink or tilt and collapse. Also liquid leakage into the float means (pontoons) of the floating roof may cause a roof to partially sink and become flooded with liquid.
For years, issues like those described above have attracted attention in the petroleum industry. There seems to be an increasing demand for systems that address the issues.
Existing monitoring of floating roofs is based on measuring relative positions or inclinations of several locations on the floating roof when in operation. There is also monitoring that combines this with, for instance, video monitoring or detection of the presence of gas on top of the floating roof.
Monitoring systems have been suggested in the past, in which relative vertical positions of several locations of the floating roof have been measured and communicated. These vertical positions are relative to a reference point of the tank or to a level of the liquid in the respective location.
It has been further suggested to provide a battery and solar cell powered monitoring system for monitoring a floating roof, in which inclination and liquid sensors are located on the roof and arranged to wirelessly communicate a status of the sensors.
It is believed that any existing or previously described monitoring systems for floating roofs have either been too complicated or otherwise lacked in ease of applicability in hazardous or potentially hazardous environments of floating roof tanks and thus have not been used in any large extent.